This invention relates to the editing and switching of digital television signals consisting of video and associated sound components. It is particularly relevant to systems which handle the audio component in compressed form.
The component parts of the TV signal necessarily carry with them information which enables their relative replay/display timing to be maintained. However, if for any reason, the replayed/displayed audio/video timings get out of step then this is most noticeable when people are speaking. Consequently, an error in audio/video replay/display timings is often referred to as xe2x80x9clip-sync errorxe2x80x9d.
A variety of techniques have been proposed for maintaining lip-sync in circumstances where the video and audio components have undergone separate processing with the introduction of different delays. Reference is directed, for example, to GB-B-2 273 215 which discloses a technique by which a delay signal is associated with either the video or audio signal, that delay signal representing the relative delay between video and audio. Signal processing elements which introduce delay are provided with synchronizing units which read the delay signal at the input, add or subtract the appropriate delay interval and insert the modified delay signal at the output. At the end of a signal processing chain, the delay signal is read and the appropriate delay interval is inserted. Since there is usually more signal processing conducted on the video component, the greater delay will usually be suffered by the video signal. The compensating delay at the end of the signal processing chain is typically an audio delay only If a video delay is required, GB-B-2 273 215 explains that this can only be adjusted in coarse steps in order to maintain synchronization between its input and output. An additional audio delay will then be used to restore precise synchronization.
The present invention recognizes that in a number of key applications, the audio signal itself will be arranged in discrete frames having a defined time interval. It will no longer be possible in such applications to achieve precise synchronizationxe2x80x94to any desired level of accuracyxe2x80x94through coarse frame-by-frame control of video delay and fine, continuous control of audio delay. The prior art proposals, when audio processing is necessarily organised into discrete audio frame intervals, will enable synchronization to be achieved only to within the resolution of one audio frame. The time intervals of audio framesxe2x80x94as will later be discussed in more detailxe2x80x94are considerably shorter than video frames and a lip-sync error of a fraction of an audio frame time interval may not be objectionable. The difficulty Is that the effect of such sub-frame synchronization errors is cumulative and conventional equipment may still produce objectionable lip-sync errors even if every delay within the chain has been defined within the accuracy of one audio frame.
It is an object of the present invention to provide improved methods for use in editing, switching and like signal processing of temporally associated frame-based signals, which address this problem.
It is a further object of this invention to provide improved methods for use in editing, switching and like signal processing where sub-frame synchronization errors are controlled or managed.
Accordingly, the present invention consists in one aspect in a method for use in the editing and switching of video and associated audio signals in which editing and switching of a video signal is constrained to video frame boundaries and editing and switching of an audio signal is constrained to audio frame boundaries, wherein accumulated sub-frame synchronization error information is carried forward.
Preferably, accumulated sub-frame synchronization error information is employed to direct a frame advance/retard decision in a frame accurate synchronization process.
Suitably, said advance/retard decision operates on audio frames.
Advantageously, accumulated sub-frame synchronization error information is used to keep the total sub-frame synchronization error introduced by the cascading of processing stages within certain tolerances.
In another aspect, the present invention consists in a method for use in editing, switching and like signal processing of temporally associated frame-based signals, in which steps are taken to ensure frame accurate synchronization, characterized in that sub-frame synchronization error feedback is provided to minimize cumulative sub-frame synchronization error.
In yet another aspect, the present invention consists in a method for use in editing, switching and like signal processing of temporally associated frame-based signals which are provided in digital form with information prescribing the time of presentation of frames, characterized by the maintenance through said processes of signal synchronization information, independent of said information prescribing the time of presentation of frames.